Polarization mode dispersion (hereinafter, “PMD”) is a signal distortion effect that can limit optical fiber transmission distances at high bit rates, such as 10 Gbits/sec (hereinafter, “Gbps”) and above. PMD is caused by variations in birefringence along the optical path that causes the orthogonal optical signal polarization modes to propagate at different velocities. The primary cause of PMD is the asymmetry of the fiber-optic strand. Fiber asymmetry may be inherent in the fiber from the manufacturing process, or it may be a result of mechanical stress on the deployed fiber. The inherent asymmetries of the fiber are fairly constant over time. In other cases the statistical nature of PMD results in unexplained PMD changes that can last for much longer periods of time, with the potential for prolonged degradation of data transmission.
Components used to split, combine, multiplex, demultiplex, amplify, reroute, or otherwise modify optical signals can also contribute to PMD.
PMD is dynamic and statistical in nature, making it a particularly difficult problem to solve. The statistical nature of PMD is such that it changes over time and varies with wavelength. Thermal and mechanical effects, such as diurnal heating and cooling, vibration from passing vehicles, fiber movement in aerial spans, and cabling disturbances by technicians e.g., during patch panel rerouting) have all been shown to cause PMD. These events can momentarily increase the PMI) in a fiber span and briefly affect the transmission quality of an optical signal. Because these effects are sometimes momentary, they are hard to isolate and diagnose. In fact, these types of problems are sometimes known as “ghosts” because they occur briefly and mysteriously, and cannot be replicated during a system maintenance window.
In long fiber spans, enough PMD can accumulate such that bits arriving at the receiver begin to interfere with another, degrading transmission quality. This effect becomes more pronounced as transmission rates get higher (and bit periods get shorter). Generally, PMD exceeding ten percent of the bit period is considered detrimental. At 10 Gbps, the bit period is 100 psecs, which implies that any span that exhibits PMD greater than 10 psecs may be “PMD-limited.” This generally only occurs in extraordinarily long spans, and those incorporating older fiber.
To date, spans deploying 10 Gbps rates have been specially-selected or “link-engineered” to low PMD fibers. As the 10 Gbps data transmission rate standard becomes more prevalent, however, PMD challenged fibers must be deployed, or lit, and specialized engineering resources may become an alternative, though cost prohibitive. Moreover, PMD is expected to be a significant and growing concern in systems transmitting information at 40 Gbps and higher. For example, at 40 Gbps, the PMD tolerance is only 2.5 psecs. At this transmission rate, every span is potentially PMD-limited.
Regeneration, inverse multiplexing, and PMD compensation are three ways of reducing the effects of PMD.
Regeneration involves, at each termination point of a span, converting the light into an electrical signal and then reconverting the electrical signal back into an optical signal for transmission along the next span. Regeneration of an optical signal is performed on each wavelength independently; meaning that each of the signals carried by a single fiber must demultiplexed, converted and reconverted, and then remultiplexed with the other wavelengths. Regeneration of optical signals was a widely used approach on all optical-transmission systems until the advent of optically amplified dense wavelength division multiplexed (“DWDM”) systems in the mid 1990's. Before that time, regenerators limited PMD and boosted the power level of the optical signal.
Once multiple wavelengths appeared on long-haul fibers, however, optical amplifiers replaced the use of regenerators for boosting signal power across multiple wavelengths. Although optical amplifiers are economical, they do not reduce PMD and may actually increase it. Therefore, optical amplification alone may not be an option on fiber spans with high PMD.
Inverse multiplexing is a second approach and is a generic term for the transport off a signal from a subscriber across multiple paths in the network at a lower bandwidth rate than it was received from the subscriber. A common example of inverse multiplexing is an application that has been around for many years in the access network: the transport of 10 Mbps Ethernet links across multiple DS-1 transmission paths. Inverse multiplexing for support of 10 Gbps services operates by disassembling a subscriber's service (e.g., an OC-192c transmission from a core router) for transport across the network by an inverse-multiplexing device. The service could be disassembled into 2.5 Gbps “chunks” for transport, then reassembled at the destination point and handed off to the destination core router. Because PMD is less of an issue at 2.5 Gbps, inverse multiplexing provides a “workaround” solution for moving 10 Gbps across a fiber network with PMD issues.
In the third approach, compensation for PMD fixes the optical signal before it is read and interpreted by the receiver at the end of the fiber path. PMD compensation methods have been explored since the potential bandwidth limitation of PMD was first recognized in the mid-1990's. Early generations of PMD compensators, however, were limited in performance, addressing only a small range of PMD.
A somewhat related type of optical distortion is chromatic dispersion (hereinafter, “CD”), which, unlike PMD, remains nearly static over time. CD causes optical pulses launched along the transmission medium to propagate at different velocities for different wavelengths of light. For example, some frequency components of a launched optical pulse will propagate slower than other frequency components, thus spreading out the pulse. Some of the methods used to compensate for CD in optical fibers are described by Ip U.S. Pat. No. 5,557,468, Ishikawa et al. U.S. Pat. No. 5,602,666, and Shigematsu et al. U.S. Pat. No. 5,701,188, all of which are hereby incorporated by reference in their entireties. Moreover, products are commercially available for providing broadband variable CD compensation (see, e.g., the dispersion compensator sold under the trademark POWERSHAPER™ by Avanex Corp. of Freemont, Calif.).
With respect to both PMD and CD, optical pulses are assumed to be bandwidth limited, and that the corresponding compensation corrects for differential delay.
Ozeki et al. describe a system that compensates delay caused by PMD in “A Polarization-Mode-Dispersion Equalization Experiment Using A Variable Equalizing Optical Circuit Controlled By A Pulse-Waveform-Comparison Algorithm,” OFC'94 Technical Digest, at 62–64 (1994), which is hereby incorporated by reference in its entirety. According to Ozeki et al., the system compensates for differential group delay (hereinafter, “DGD”) by subjecting a distorted optical signal to a polarization transformation, transmitting the transformed signal through a birefringent fiber, subjecting the transmitted signal to one or two additional polarization transformations, and transmitting the transformed signal through another birefringent fiber. Patscher et al. describes another compensation scheme similar to Ozeki et al. in “A Component For Second-Order Compensation Of Polarisation-Mode Dispersion” in Electronics Letters, Vol. 33, No. 13., at 1157–1159 (Jun. 19, 1997). Neither publication, however, describes how the polarization state of an optical signal is transformed.
Fishman et al. U.S. Pat. No. 5,930,414, which is hereby incorporated by reference in its entirety, describes a system for compensating first-order PMD. Because PMD is dynamic, the system shown by Fishman et al. adaptively compensates for DGD by varying the orientation of a birefringence element.
The apparatus shown by Fishman et al. includes a polarization transformer coupled in series with a birefringence element. The distorted optical signal is input to the polarization transformer. The birefringence element provides a compensated optical signal, which is optically tapped and converted by a photodetector into an electrical signal. The electrical signal is then amplified and the distortion in the amplified photocurrent is measured by a distortion analyzer that generates a control voltage in accordance with the measured distortion. The distortion analyzer outputs a control voltage that approaches a maximum value when distortion in the optical signal due to first order PMD approaches a minimum. The control voltage is provided as feedback to the polarization transformer and the birefringence element in a feedback loop. The polarization transformer and the birefringence element are thus continually varied via feedback control to compensate for optical distortion resulting from PMD.
It is known that purely optical PMD compensation can be performed using an RF spectrum feedback. There are, however, several complicating factors that make RF feedback techniques difficult to execute. For example, the PMD spectrum at and near the data bit period can be ambiguous because of strong inter-symbol interference effects of adjacent pulses. Also, second order PMD tends to complicate the compensation process for at least two reasons: (1) the reduced sensitivity of the spectral intensity as a function of DGD and (2) the non-monotonic behavior of the spectrum.
Purely electronic PMD feedback techniques are also problematic. Electronic equalizers, for example, normally rely on error detection to generate proper feedback signals. Accurate error detection, however, is very difficult when a large amount of distortion is present. Moreover, each gain stage of an electronic equalizer must maintain its linearity over supply voltage and temperature ranges, as well as any processing variations, which are substantial challenges for high data rate applications.
Thus, it is difficult to totally eliminate PMD and CD using optical compensators or electronic equalizers alone, especially at data transmission rates of 10 Gpbs or more.
It would therefore be desirable to provide methods and apparatus for adaptive optical dispersion compensation for data transmission rates of 10 Gbps or more, particularly using both optical and electronic compensation means.
It would also be desirable to provide integrated apparatus and methods for adaptively compensating for dispersion impairment due to PMD, CD and the like, thereby enabling high-speed optical data transfer with minimal data transmission errors.